Gerrit » History » Revision 54

Revision 53 (neels, 09/26/2016 11:37 PM) → Revision 54/130 (neels, 10/13/2016 02:40 AM)

h1. Contributing using Gerrit 


 At [[OpenBSC:OsmoDevCon2016]] we discussed problems with our past contribution / patch submission process using mails on the mailing list as well as patchwork.    The result is that we want to give Gerrit a try for some time and see if it helps us to have a better process 

 Gerrit is a review tool that integrates nicely with git and ssh. You can find general information about Gerrit at 

 The advantages of Gerrit are: 
 * patch submission status is automatically tracked, also with several revisions for a patch set. 
 * patches are build-tested (and possibly even further tested) by jenkins before they are applied 
 * developers + maintainers can formally vote on a patch (developer: -1/0/+1, maintainer: -2/0/+2) 
 * once a patch has +2 score, it can be (automatically) merged into master 
 * patch sumissions not via git send-email but direcly from git 

 h2. Osmocom Subprojects using Gerrit 

 The following projects use Gerrit to contribute changes: 

 * libosmocore.git 
 * libosmo-abis.git 
 * libosmo-netif.git 
 * libosmo-sccp.git 
 * libsmpp34.git 
 * openbsc.git 
 * osmo-bts.git 
 * osmo-iuh.git 
 * osmo-pcu.git 
 * cellmgr-ng.git 
 * osmo-sip-connector.git 


 h2. Configuring Gerrit/Account 

 You will need to sign-up at If you have an Osmocom Redmine account you can use as OpenID provider. 

 * first sign in on 
 * go to and klick "Sign in" 
 * enter 

 If you have no Osmocom redmine account, you can simply create one online at the "Register" link in the upper right corner. 

 Even without an existing or new redmine account, you should also be able to use any other OpenID provider to authenticate against gerrit (untested). 

 After the initial sign-up you will need to: 

 * Pick a username (can not be changed) 
 * Add your public ssh key(s) 
 * Add email addresses you intend to use as author/comitter 

 If you would like to push private branches to the Gerrit repository, you also need to be added to the "known users" group. 
 Please send a short requesting email to 


 h2. Setting up Gerrit for commits and pushing 

 *Note:* it is easiest to work with gerrit when gerrit is the only remote in your git clone. 
 When you clone from and add the gerrit remote, git will have two remotes, 
 so when you first checkout a branch you have to supply the remote explicitly (cumbersome). 
 The gerrit repositories and are constantly synced, so it is sufficient 
 to clone from gerrit only. 

 h3. Simplest: new clone 

 * Create a new clone from gerrit 
 * Fetch the commit hook that adds Change-Id to each commit to uniquely identify a commit 

 git clone ssh://$$PROJECT.git 
 scp -P 29418 $ $PROJECT/.git/hooks/ 

 h3. SSH config 

 In '~/.ssh/config', add these lines: 
 Host go 
 Port 29418 
 ('go' means gerrit.osmocom, replace with your favorite shortcut name, 
 replace '$USERNAME' with your user name as used on the gerrit website) 

 Then you can shorten above commands to 
 git clone ssh://go/$PROJECT.git 
 scp go:hooks/commit-msg .git/hooks/ 

 h3. Committer must match 

 Your email address on gerrit and the email address git places in your 
 commits must match, or you will get rejected with an error message like 
 "invalid commiter". You can add email addresses on the gerrit web UI. 

 h3. Add gerrit to an existing clone 

 * Add the remote to be able to fetch and push to gerrit 
 * Fetch the commit hook that adds Change-Id to each commit to uniquely identify a commit 

 PROJECT=$(basename $PWD) 
 git remote add gerrit ssh://$$PROJECT.git 
 scp -P 29418 $ .git/hooks/ 

 h2. Push for review 

 Checkout the revision or branch that you want to submit for review, then 

 git push gerrit HEAD:refs/for/master 

 You can optionally add a topic name with 

 git push gerrit HEAD:refs/for/master/my_topic 

 h2. Push a "private" user branch 

 *Note* that you must be a member of the "known users" group, see above. 

 If your local branch name is of the form 'your_name/topic', you can just 
 git push 
 and git will tell you what to do. 

 To push from a "nonstandard" local branch name, do 
 git push gerrit HEAD:refs/heads/user/$USERNAME/branch_name 

 h2. List changesets in gerrit 

 git ls-remote gerrit changes/* 

 h1. Tips and Tricks 

 h2. Throw-away branch 

 If you need to adjust and re-submit patches, it may be handy to create a throw-away branch ("R D" in magit-gerrit in emacs for example), 
 make your changes/amendments and then send patch(es) back to gerrit while removing temporary branch automatically with "git review -f". 

 h2. Re-submit a Branch with Amended Commits 

 On a feature branch, one typically has numerous commits that depend on their preceding commits. 
 Often, some of the branch commits need to be amended for fixes. But, Gerrit will refuse your branch 
 re-submission if the first branch commit is unchanged. 

 To re-submit a branch, make sure to cosmetically tweak the branch's first commit log message 
 before each re-submission (keep the Change-Id, really make just a cosmetic change). 

 git rebase -i master 
 # replace the first line's 'pick' with 'r' (or 'reword'), exit editor 
 # git presents you with commit log message, make any tiny modification. 

 The cause: Gerrit refuses to accept a commit with a Change-Id that it already knows and 
 where the commit hash is identical. 

 If you just cosmetically tweak the first commit's log message, the commit hash 
 is changed. Since the following commits contain their predecessor's commit hash, now 
 all of the branch's commit hashes are modified, and gerrit happily accepts them as a  
 new patch set. It will still pick up the Change-Ids (which you shouldn't edit) and  
 notice if commits have remained identical (keeping the votes). But with the minor 
 commit log tweak, it will no longer thwart your re-submission with an error message. 

 Note: you could modify all the Change-Ids, but now your branch submission would 
 open entirely new review entries and you would have to abandon your previous submission. 
 Comments on the first submission are lost and you cannot diff between patch sets. 

 h2. Re-submit Previously Abandoned Changes 

 You have to edit the Change-Ids, on a branch that would be every single commit log message. 

 cd openbsc 
 git co my-branch 
 git rebase -i master 
 # replace all 'pick' with 'r' (or 'reword'), exit your editor 
 # git presents each commit log message for editing 

 h2. Submit a "private" branch for master 

 If you've pushed a branch to refs/heads/* somewhere, gerrit will already know the Change-Ids on it. 
 Make sure the option [[Gerrit#Private-Branches-Create-a-new-change-for-every-commit|Create a new change for every commit not in the target branch]] is _TRUE_ for your project, 
 or gerrit will refuse to accept your submission. 

 h1. Reasons for Particular Configuration 

 h2. Rebase if necessary 

 There are different merge strategies that Gerrit performs to accept patches. 
 Each project can be configured to a specific merge strategy, but unfortunately you can't 
 decide on a strategy per patch submission. 

 It seems that the "Merge if Necessary" strategy is best supported, but it creates non-linear 
 history with numerous merge commits that are usually not at all necessary. 

 Instead, the "Cherry Pick" strategy puts each patch onto current master's HEAD to create 
 linear history. However, this will cause merge failures as soon as one patch depends on 
 another submitted patch, as typical for a feature branch submission. 

 So we prefer the "Rebase if Necessary" strategy, which always tries to apply your patches to 
 the current master HEAD, in sequence with the previous patches on the same branch. 
 However, some problems still remain, including some bugs in "Rebase if Necessary". 

 There's a problem with "Rebase if Necessary": If your branch sits at master's HEAD, Gerrit 
 refuses to accept the submission, because it thinks that no new changes are submitted. 
 This is a bug in Gerrit, which holger has fixed manually in our Gerrit installation: 

 h2. Private Branches: Create a new change for every commit... 

 Say you have an extensive feature in development, and you want to keep it on the 
 upstream git repository to a) keep it safe and b) collaborate with other devs on it. 
 So, of course, you have regularly pushed to refs/heads/yoyodyne/feature. 

 Since you have the gerrit commit hook installed, your feature branch already has 
 Change-Id tags in all commit log messages. 

 Now your feature is complete and you would like to submit it to master. 
 Alas, Gerrit refuses to accept your patch submission for master, because it 
 knows the Change-Ids are also on a different branch. 

 Gerrit by default enforces that a Change-Id must be unique across all branches, 
 so that each submission for review is separate for each branch. Instead, we 
 want to handle Change-Ids per-branch, so that you can have the same change 
 submitted to different branches, as separate patch submissions, without having 
 to cosmetically adjust the Change-Id. 

 Solution: set the option  
 _Create a new change for every commit not in the target branch_ to _TRUE_ 

 h2. Allow content merges 

 By default, gerrit compares patches only by the files' paths. If two paths are the same, 
 it immediately shows them as conflicts (path conflicts). 

 In software development, a conflict usually means an actual content conflict, so if the 
 edits are in two entirely separate places in the file, we don't consider this a conflict. 

 By setting _Allow content merges_ to _TRUE_ in the git project config, we tell Gerrit to 
 perform text merges of the submitted patches and only complain about actual content 
 conflicts, in the usual software engineering sense. 

 h1. Admin 

 h2. Adding users to groups 

 Normally, the gerrit UI auto-completes a user name in the edit field. It has happened 
 though that an existing user is not auto-completed, as if it didn't exist. In that case, 
 find out the user ID (seven digit number like 1000123) and just enter that. 

 The user ID can be found on the user's "Settings" page, or in the database (s.b.). 

 h2. Querying the database directly 

 If your user has permission to access the database, you can place SQL queries using the 
 'gerrit gsql' commands over ssh: 

 ssh go 'gerrit gsql --format PRETTY -c "show tables"' 
 ssh go 'gerrit gsql --format PRETTY -c "select full_name,account_id from accounts"' 

 (see ~/.ssh/config above for the 'go' shortcut) 

 This seems to be the MySQL dialect.
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